1. Field of the Invention
The present invention broadly relates to the production of a monocrystalline rod by Czochralski method and, more particularly, to an apparatus which is capable of measuring, by processing image signal from an image sensor, the diameter of a monocrystalline at the surface of a melt during growth of the crystal by Czochralski method.
2. Description of the Prior Art
In general, a silicon monocrystalline rod is ground until its diameter is reduced to the final size before the rod is sliced into wafers. Although the grinding margin is preferably small, there is a practical limit in the reduction of the grinding margin due to various reasons.
In recent years, a method known as Czochralski method is finding spreading use in the production of monocrystalline rod. Silicon monocrystalline rods produced by this method generally have large diameters, say 158 mm or so and, therefore, the volume of the rod to be removed by grinding with respect to the grinding margin is correspondingly large, with the result that the production cost is raised uneconomically. It would be worth saying that, when the monocrystalline rod has a length of 600 mm and a diameter of 158 mm, the amount to be removed by grinding is as large as 298 cm.sup.3 even when the grinding margin is as small as 1 mm in terms of diameter. Thus, reduction in the grinding margin of the monocrystalline rod is a matter of great significance, considering the current increasing demand for lower production cost.
The grinding margin would be reduced appreciably if the irregularity of the rod diameter could be reduced by attaining higher response and stability in the diameter control operation. A higher degree of accuracy in the measurement of the monocrystalline rod diameter also would contribute to the reduction in the grinding margin.